Community
and Population Ecology

1987, this reclassification was extended to seven more states.
Currently, Florida, Louisiana, Texas, and Georgia allow alligator
hunting. Today there are 1–2 million alligators in Florida and the
state now allows property owners to kill rogue alligators on their
property instead of filing a request for professional help.

The recent increase in demand for alligator meat and hides

has created a booming business for alligator farms, especially in
Florida. Such farms reduce the need for illegal hunting of wild
alligators.

To biologists, the comeback of the American alligator is an

important success story in wildlife conservation. Its tale illustrates
how each species in a community or ecosystem fills a unique
role, and it highlights how interactions between species can af-
fect ecosystem structure and function. In this chapter, we will
look at how species interact and how biological communities and
populations respond to changes in environmental conditions.

Why Should We Care about the
American Alligator?

The American alligator (Figure 6-1), North America’s largest rep-
tile, has no natural predators except for humans and plays a
number of important roles in the ecosystems where it is found.
This species, which has been around for nearly 200 million years,
has outlived the dinosaurs. It has been able to adapt to numer-
ous changes in the earth’s environmental conditions.

This changed when hunters began killing large numbers of

these animals for their exotic meat and their supple belly skin,
used to make shoes, belts, and pocketbooks. Other people
hunted alligators for sport or out of hatred. Between l950 and
1960, hunters wiped out 90% of the alligators in the U.S. state
of Louisiana. By the 1960s, the alligator population in the Florida
Everglades was also near extinction.

People who say “So what?” are overlooking the alligator’s im-

portant ecological role—its niche (

Concept 4-3

, p. 68)

—in subtropical wetland communities. Alligators dig
deep depressions, or gator holes, which hold freshwater during
dry spells, serve as refuges for aquatic life, and supply freshwater
and food for many animals. Large alligator nesting mounds pro-
vide nesting and feeding sites for species of herons and egrets.
Alligators eat large numbers of gar, a predatory fish. This helps
maintain populations of game fish such as bass and bream.

As alligators move from gator holes to nesting mounds, they

help keep areas of open water free of invading vegetation.
Without these free ecosystem services, freshwater ponds and
coastal wetlands found where alligators live would be filled in
with shrubs and trees, and dozens of species would disappear
from these ecosystems.

Some ecologists classify the American alligator as a keystone

species because of its important ecological role in helping main-
tain the structure, function, and sustainability of the communities
where it is found.

In 1967, the U.S. government placed the American alligator

on the endangered species list. Protected from hunters, the pop-
ulation made a strong comeback in many areas by 1975—too
strong, according to those who find alligators in their backyards
and swimming pools, and to duck hunters whose retriever dogs
are sometimes eaten by alligators. Since 1948, alligators have
killed about 20 people in Florida.

In 1977, the U.S. Fish and Wildlife Service reclassified the

American alligator as a threatened species in the U.S. states of
Florida, Louisiana, and Texas, where 90% of the animals live. In

C O R E C A S E S T U D Y

6

A. & J. V

isage/Peter Arnold, Inc.

Figure 6-1 The American alligator plays an important ecological role
in its marsh and swamp habitats in the southeastern United States.
Since being classified as an endangered species in 1967, it has recovered
enough to have its status changed from endangered to threatened—an
outstanding success story in wildlife conservation.

83376_07_ch06_p105-122.ctp 8/10/07 12:17 PM Page 105

106

Key Questions and Concepts

6-1

How Does Species Diversity Affect the

Sustainability of a Community?

C O N C E P T 6 - 1

Species diversity is a major component of bio-

diversity and tends to increase the sustainability of communities
and ecosystems.

6-2

What roles do species play in a community?

C O N C E P T 6 - 2

Based on certain ecological roles they play in

communities, species are described as native, nonnative, indicator,
keystone, or foundation species.

6-3

How do species interact?

C O N C E P T 6 - 3 A

Five basic species interactions—competition,

predation, parasitism, mutualism, and commensalism—affect the
resource use and population sizes of the species in a community.

C O N C E P T 6 - 3 B

Some species develop adaptations that allow

them to reduce or avoid competition for resources with other
species.

6-4

How do communities respond to changing

environmental conditions?

C O N C E P T 6 - 4 A

The structure and species composition of

communities change in response to changing environmental
conditions through a process called ecological succession.

C O N C E P T 6 - 4 B

According to the precautionary principle,

we should take measures to prevent or reduce harm to human
health and natural systems even if some possible cause-and-effect
relationships have not been fully established scientifically.

6-5

What limits the growth of populations?

C O N C E P T 6 - 5

No population can continue to grow indefinitely

because of limitations on resources and because of competition
among species for those resources.

Animal and vegetable life is too complicated a problem

for human intelligence to solve,

and we can never know how wide a circle of disturbance we produce

in the harmonies of nature when we throw

the smallest pebble into the ocean of organic life.

GEORGE PERKINS MARSH

What Is Species Diversity?

Recall that a community is a collection of populations of
different species in a given area that can potentially in-
teract with one another. Biological communities differ
in the types and numbers of species they contain and
the ecological roles those species play (

Con-

cept 4-3

, p. 68). These communities are shaped

by the species they contain and by feeding relationships
and other interactions among those species. An impor-
tant characteristic of a community is its species diver-
sity: the number of different species it contains
(species richness) combined with the relative abun-
dance of individuals within each of those species
(species evenness).

For example, a biologically diverse community such

as a tropical rain forest or a coral reef with a large num-

ber of different species (high species richness) generally
has only a few members of each species (low species
evenness). Biologist Terry Erwin found an estimated
1,700 different beetle species in a single tree in a tropi-
cal forest in Panama but only a few individuals of each
species. On the other hand, an evergreen forest com-
munity in the U.S. state of Alaska may have only ten
plant species (low species richness) but large numbers
of each species (high species evenness).

Such species diversity is one of the major com-

ponents of biodiversity (Figure 3-12, p. 48,
and

Concept 3-4A

, p. 48). Another commu-

nity characteristic is its niche structure: how many eco-
logical niches occur, how they resemble or differ from
one another, and how the species occupying different
niches interact (

Concept 4-3

, p. 68).

6-1

How Does Species Diversity Affect the Sustainability
of a Community?

C O N C E P T 6 - 1

Species diversity is a major component of biodiversity and tends to increase

the sustainability of communities and ecosystems.

Note: Supplements 4, 6, 9, and 11 can be used with this chapter.

Links:

refers to the Core Case Study.

refers to the book’s sustainability theme.

indicates links to key concepts in earlier chapters.

83376_07_ch06_p105-122.ctp 8/10/07 12:17 PM Page 106

Is this a valid hypothesis? Because no community

can function without some producers and decomposers
(

Concept 3-3

, p. 44), there is a minimum

threshold of species diversity below which
communities and ecosystems either cannot function or
function poorly. Many studies support the idea that
some level of species diversity provides insurance
against catastrophe. But how much species richness is
needed to help sustain various communities remains
uncertain.

Some recent research suggests that the average an-

nual NPP of an ecosystem reaches a peak with 10–40
producer species. Many ecosystems contain more than
40 producer species, but it is difficult to distinguish
among those that are essential and those that are not.
At any rate, communities vary in their likely level of
sustainability, related in some way to differences in
species richness and the ecological roles played by their
species (

Concept 6-1

). While there may be some excep-

tions to this idea, most ecologists now accept it as a
useful hypothesis. The Science Focus above sheds more
light on this issue.

RESEARCH FRONTIER

Learning more about the relationship between species diver-
sity and sustainability in communities

THINKING ABOUT

The American Alligator’s Niche

Does the American alligator (

Core Case Study

) have

a specialist or a generalist niche? Explain.

The species diversity of communities varies with

their geographical location. For most terrestrial plants and
animals, species diversity (primarily species richness) is
highest in the tropics and declines as we move from
the equator toward the poles. The most species-rich
environments are tropical rain forests, coral reefs, the
ocean bottom zone, and large tropical lakes—most of
them under severe and increasing pressure from hu-
man activities.

Learn about how latitude affects species diver-

sity and about the differences between big and small islands at
ThomsonNOW.

Sustainability Involves Resisting
or Responding to Changing
Environmental Conditions

All living systems, from a cell to the biosphere (Fig-
ure 3-3, p. 41), maintain some degree of sustainability
or stability by constantly changing in response to
changing environmental conditions. It is useful to dis-
tinguish among three aspects of stability or sustainabil-
ity in living systems. One is inertia, or persistence: the
ability of a living system to resist being disturbed or al-
tered. A second is constancy: the ability of a living sys-
tem such as a population to keep its numbers within the
limits imposed by available resources. A third factor is
resilience: the ability of a living system to repair dam-
age after an external disturbance that is not too drastic.

Species-Rich Communities
Tend to Be Productive
and Sustainable

Does a community with a high species richness tend
to have greater sustainability and productivity than
one with a lower species richness? Is a species-rich
community better able to recover or “bounce back”
from, say, a drought than a community that is not as
diverse? Research suggests that the answers to both
questions may be yes, but more research is needed be-
fore these scientific hypotheses can be accepted as sci-
entific theories.

According to the first hypothesis, a complex commu-

nity with many different species (high species richness)
and the resulting variety of feeding paths has more ways
to respond to most environmental stresses because it
does not have “all its eggs in one basket.”

CONCEPT 6-1

107

S C I E N C E F O C U S

Community Sustainability:
A Closer Look

cologists disagree on how to define sustainability or stability.
For example, does a community need both high inertia and

high resilience to be considered sustainable?

Evidence suggests that some communities have one of these properties

but not the other. Tropical rain forests have high species richness and high
inertia and thus are resistant to significant alteration or destruction. But
once a large tract of tropical forest is severely degraded, the community’s
resilience may be so low that the forest may not be restored. Nutrients
(which are stored primarily in the vegetation, not in the soil), and other
factors needed for recovery may no longer be present. Such a large-scale
loss of tropical forest cover may also change the local or regional climate
so that forests can no longer be supported.

By contrast, grasslands have a much lower species richness than most

forests and have low inertia because they burn easily. However, because
most of their plant matter is stored in underground roots, these ecosys-
tems have high resilience and recover quickly. Grassland can be destroyed
only if its roots are plowed up and something else is planted in its place, or
if it is severely overgrazed by livestock or other herbivores.

Another difficulty is that populations, communities, and ecosystems are

rarely, if ever, at equilibrium. Instead, nature is in a continuing state of dis-
turbance, fluctuation, and change.

Critical Thinking

Are deserts fairly sustainable communities? Explain.

E

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108

CHAPTER 6

Community and Population Ecology

Niches Can Be Occupied
by Native and Nonnative
Species

Each species in a community occupies a unique ecological
niche that describes its role in a community. It includes
the particular habitat in which it lives, the environ-
mental conditions such as temperature (Figure 3-9,
p. 45) necessary for its survival, and the methods it uses
to acquire its supply of nutrients.

Ecologists focus on the different ecological roles or

niches that native, nonnative, indicator, keystone, and
foundation species play in communities (

Concept 6-2

).

Any given species may play more than one of these
ecological roles in a particular community.

Native species are those species that normally live

and thrive in a particular community. Other species
that migrate into a community, or are deliberately or
accidentally introduced, are called nonnative species,
invasive species, or alien species. Some people tend
to think of nonnative species as villains. In fact, most
introduced and domesticated species of crops and ani-
mals such as chickens, cattle, and fish from around the
world are beneficial to us.

Sometimes, however, a nonnative species can re-

duce some or most of a community’s native species and
cause unintended and unexpected consequences. In
1957, for example, Brazil imported wild African bees to
help increase honey production. Instead, the bees dis-
placed domestic honeybees and reduced the honey
supply. Since then, these nonnative bees—popularly
known as “killer bees”—have moved northward into
Central America and parts of the southwestern United
States.

The wild African bees are not the fearsome killers

portrayed in some horror movies, but they are aggres-
sive and unpredictable. They have killed thousands of
domesticated animals and an estimated 1,000 people in
the western hemisphere, many of whom were allergic
to bee stings or because they fell down or became
trapped and could not flee.

Nonnative species can spread rapidly if they find

new niches that are as suitable as their original niches
were. In their new niches, these species often do not
face predators and diseases they had before, or they
may be able to out-compete some native species in
their new niches. We will examine this environmental
problem in greater detail in Chapter 9.

Indicator Species Are
Biological Smoke Alarms

Species that provide early warnings of harmful environ-
mental changes taking place in a community or an
ecosystem are called indicator species. For example,
the presence or absence of trout species in water at tem-
peratures within their range of tolerance (Figure 3-9,
p. 45) is an indicator of water quality because trout need
clean water with high levels of dissolved oxygen.

Birds are excellent biological indicators because

they are found almost everywhere and are affected
quickly by environmental changes such as loss or frag-
mentation of their habitats and introduction of chemi-
cal pesticides. The populations of many bird species are
declining. Butterflies are also good indicator species be-
cause their association with various plant species makes
them vulnerable to habitat loss and fragmentation.
Some amphibians are also classified as indicator species
(Case Study, below).

■

C A S E S T U D Y

Why Are Amphibians Vanishing?

Amphibians (frogs, toads, and salamanders) live part of
their lives in water and part on land, and some are
classified as indicator species. Frogs, for example, are
especially vulnerable to environmental disruption at
various points in their life cycle, shown in Figure 6-2.

As tadpoles, they live in water and eat plants; as

adults, they live mostly on land and eat insects that can
expose them to pesticides. Frogs’ eggs have no protec-
tive shells to block ultraviolet (UV) radiation or pollu-
tion. As adults, they take in water and air through their
thin, permeable skins, which can readily absorb pollu-
tants from water, air, or soil.

Since 1980, populations of hundreds of the world’s

almost 6,000 amphibian species have been vanishing or
declining in almost every part of the world, even in pro-
tected wildlife reserves and parks. According to the
2004 Global Amphibian Assessment, about 33% of all
known amphibian species are threatened with extinc-
tion, and populations of 43% of the species are declin-
ing—a catastrophic loss of biological diversity.

No single cause has been found to explain the am-

phibian declines. However, scientists have identified a
number of factors that can affect frogs and other am-
phibians at various points in their life cycles:

6-2

What Roles Do Species Play
in a Community?

C O N C E P T 6 - 2

Based on certain ecological roles they play in communities, species are de-

scribed as native, nonnative, indicator, keystone, or foundation species.

83376_07_ch06_p105-122.ctp 8/10/07 12:17 PM Page 108

•

Habitat loss and fragmentation (especially from drain-
ing and filling of inland wetlands, deforestation,
and development).

•

Prolonged drought (which dries up breeding pools so
few tadpoles survive).

•

Pollution (particularly from exposure to pesticides,
which can make frogs more vulnerable to bacterial,
viral, and fungal diseases and can cause sexual
abnormalities).

•

Increases in ultraviolet radiation caused by reductions
in stratospheric ozone (which can harm embryos
of amphibians in shallow ponds).

•

Parasites (organisms that feed on amphibians).

•

Viral and fungal diseases (especially a fungus that
attacks the skin of frogs).

•

Climate change. Global warming evaporates water
and increases cloud cover in tropical forests. This
lowers daytime temperatures, making nights
warmer and creating conditions favorable for the
spread of a skin fungus deadly to frogs. It can also
dry up frog habitat, which led to the extinction of
Costa Rica’s golden toad (Figure 4-7, p. 70).

•

Overhunting (especially in Asia and France, where
frog legs are a delicacy).

•

Natural immigration or deliberate introduction of nonna-
tive predators and competitors (such as fish).

A combination of such factors probably is responsible for
the decline or disappearance of most amphibian species.

Why should we care if some amphibian species be-

come extinct? Scientists give three reasons. First, this

trend suggests that environmental health is deteriorat-
ing in parts of the world because amphibians are sensi-
tive biological indicators of changes in environmental
conditions such as habitat loss and degradation, pollu-
tion, UV radiation exposure, and climate change.

Second, adult amphibians play important ecological

roles in biological communities. For example, amphib-
ians eat more insects (including mosquitoes) than do
birds. In some habitats, extinction of certain amphibian
species could lead to extinction of other species, such
as reptiles, birds, aquatic insects, fish, mammals, and
other amphibians that feed on them or their larvae.

Third, amphibians represent a genetic storehouse of

pharmaceutical products waiting to be discovered.
Compounds in secretions from amphibian skin have
been isolated and used as painkillers and antibiotics
and as treatment for burns and heart disease.

The plight of some amphibian indicator species is a

warning signal. They may not need us, but we and
other species need them.

THINKING ABOUT

Amphibians

List three ways in which your lifestyle could be contributing
to the decline of some amphibian species.

Keystone Species Play Important
Roles in Communities

A keystone is the wedge-shaped stone placed at the top
of a stone archway. Remove this stone and the arch col-
lapses. In some communities, ecologists hypothesize

CONCEPT 6-2

109

Figure 6-2 Typical life
cycle of a frog. Popula-
tions of various frog
species can decline be-
cause of the effects
of harmful factors at
different points in their
life cycle. Such factors
include habitat loss,
drought, pollution, in-
creased ultraviolet radia-
tion, parasitism, disease,
overhunting, and non-
native predators and
competitors.

Young frog

Adult frog
(3 years)

Young frog

Adult frog
(3 years)

Tadpole
develops
into frog

Tadpole

Egg hatches

Fertilized egg
development

Sexual
reproduction

Sperm

Eggs

Organ formation

83376_07_ch06_p105-122.ctp 8/10/07 12:17 PM Page 109

■

✓

that keystone species serve a similar role by having a
much larger effect than their numbers would suggest on
the types and abundances of other species in those com-
munities. Eliminating a keystone species may dramati-
cally alter the structure and function of a community.

Keystone species play critical ecological roles. One

is pollination of flowering plant species by bees, butter-
flies (Figure 3-1, right, p. 38), hummingbirds, bats, and

other species. In addition, top predator keystone species
feed on and help regulate the populations of other
species. Examples are the wolf, leopard, lion, alligator
(

Core Case Study

), and some shark species

(Science Focus, above).

HOW WOULD YOU VOTE?

Do we have an ethical obligation to protect shark species from
premature extinction and treat them humanely? Cast your
vote online at www.thomsonedu.com/biology/miller.

Have you thanked a dung beetle today? Perhaps you

should. These keystone species (Figure 6-3) rapidly re-
move, bury, and recycle dung. They also churn and
aerate soil, making it more suitable for plant life.
Without them, in many places we would be up to our
eyeballs in animal wastes and many plants would be
starved for nutrients.

The loss of a keystone species can lead to population

crashes and extinctions of other species in a community
that depends on it for certain ecological services. This
explains why it is so important for scientists to identify
and protect keystone species.

110

CHAPTER 6

Community and Population Ecology

Figure 6-3 A keystone
species: this dung bee-
tle has rolled up a ball of
fresh dung. They roll the
balls into tunnels where
they have laid eggs.
When the eggs hatch,
the larvae have an easily
accessible food supply.
These hardworking recy-
clers play keystone roles
in many communities.

Michael Rauch/Peter Arnold, Inc.

S C I E N C E F O C U S

Why Should We Protect Sharks?

he world’s 370 shark species vary
widely in size. The smallest is the

dwarf dog shark, about the size of a large
goldfish. The largest, the whale shark, can
grow to 15 meters (50 feet) long and weigh
as much as two full-grown African elephants.

Shark species, feeding at the tops of food

webs (

Concept 3-5

, p. 50), remove

injured and sick animals from the
ocean, and thus play an important ecological
role. Without their services, the oceans would
be teeming with dead and dying fish.

Many people—influenced by movies and

popular novels—think of sharks as people-
eating monsters. In reality, the three largest
species—the whale shark, basking shark, and
megamouth shark—are gentle giants. They
swim through the water with their mouths
open, filtering out and swallowing huge
quantities of plankton.

Media coverage of shark attacks greatly

distorts the danger from sharks. Every year,
members of a few species—mostly great
white, bull, tiger, gray reef, lemon, hammer-
head, shortfin mako, and blue sharks—injure
60–100 people worldwide. Since 1990,
sharks have killed an average of seven people
per year. For risk comparison purposes,
poverty kills about 11 million people a year,

tobacco 5 million a year, and air pollution 3
million a year.

For every shark that injures a person,

humans kill at least 1 million sharks. Sharks
are caught mostly for their valuable fins and
are often thrown back alive into the water,
fins hacked off, where they bleed to death or
drown because they can no longer swim. The
fins are widely used in Asia as a soup ingredi-
ent and as a pharmaceutical cure-all. A top
(dorsal) fin from a large whale shark can fetch
up to $10,000. In high-end restaurants in
China, a bowl of shark fin soup can cost
$100 or more. Ironically, shark fins have been
found to contain dangerously high levels of
toxic mercury.

Sharks are also killed for their livers, meat,

hides, and jaws, and because we fear them.
Declining fish stocks in some parts of the
world has lead to increased fishing of sharks
for their meat. Some sharks die when they
are trapped in nets or lines deployed to catch
swordfish, tuna, shrimp, and other species.
And overfishing threatens about one of every
five shark species.

Sharks might save human lives if we can

learn from them how to fight cancer, which
they almost never get. Scientists are also
studying their highly effective immune sys-

tem, which allows wounds to heal without
becoming infected.

Sharks are especially vulnerable to over-

fishing because they grow slowly, mature late,
and have only a few young each generation.
Today, they are among the most vulnerable
and least protected animals on earth.

In 2003, experts at the National Aquarium

in the U.S. city of Baltimore, Maryland, esti-
mated that populations of some shark species
have decreased by 90% since 1992. Eight
of the world’s shark species are considered
critically endangered or endangered and
82 species are threatened with extinction.

In response to a public outcry over deple-

tion of some species, the United States and
several other countries have banned hunting
sharks for their fins in their territorial waters.
But such bans are difficult to enforce.

Sharks have been around for more than

400 million years. Sustaining this portion of
the earth’s biodiversity by preserving these
keystone species begins with the knowledge
that sharks may not need us, but we and
other species need them.

Critical Thinking

What are three things you would do to help
protect sharks from premature extinction?

T

83376_07_ch06_p105-122.ctp 8/10/07 12:18 PM Page 110

THINKING ABOUT

The American Alligator

What species might disappear or suffer sharp popu-
lation declines if the American alligator (

Core Case Study

)

became extinct in subtropical wetland ecosystems?

Foundation Species Also Play
Important Ecological Roles

Another important type of species in some communi-
ties is a foundation species, which plays a major role
in shaping communities by creating and enhancing
their habitats in ways that benefit other species. For ex-
ample, elephants push over, break, or uproot trees, cre-
ating forest openings in the savanna grasslands and
woodlands of Africa. This promotes the growth of
grasses and other forage plants that benefit smaller

grazing species such as antelope. It also accelerates nu-
trient cycling rates.

Some bat and bird foundation species can regener-

ate deforested areas and spread fruit plants by deposit-
ing plant seeds in their droppings. Beavers acting as
“ecological engineers” create wetlands used by other
species. They do this by felling trees along shorelines
and using them to build dams across streams, which
serve as their lodge homes.

In general, the main difference between keystone

and foundation species is that foundation species, such
as beavers, help create habitats and ecosystems. A foun-
dation species thus strengthens and sometimes expands
the foundation of its community.

RESEARCH FRONTIER

Identifying, studying, and protecting keystone and foundation
species

CONCEPTS 6-3A AND 6-3B

111

Most Species Compete
with One Another for Resources

When different species in a community have activities
or resource needs in common, they may interact with
one another. Members of these species may be harmed,
helped, or unaffected by the interaction. Ecologists
identify five basic types of interactions between species:
interspecific competition, predation, parasitism, mutualism,
and commensalism.

These interactions have profound effects on the re-

source use and population sizes of species in a commu-
nity (

Concept 6-3A

). They influence the abilities of the

interacting species to survive and reproduce, and thus
the interactions serve as agents of natural selection
(

Concept 4-1B

, p. 64). Some interactions also

help limit population sizes, illustrating one of
the four

scientific principles of sustainability

(see

back cover).

The most common interaction between species is

competition for shared or limited resources such as space
and food. Ecologists call such competition between
species interspecific competition. No two species
can share the same vital and limited resource for very
long. When intense competition for resources such as
food, sunlight, water, and nesting sites occurs, one of
the competing species must migrate to another area (if

possible), shift its feeding habits or behavior through
natural selection, suffer a sharp population decline, or
become extinct in that area.

Some Species Evolve Ways
to Share Resources

Over a time scale long enough for natural selection to
occur, populations competing for the same resources
develop adaptations that allow them to reduce or avoid
competition for resources with other species (

Con-

cept 6-3B

). One way this happens is through resource

partitioning. It occurs when species competing for
similar scarce resources evolve specialized traits that al-
low them to use shared resources at different times, in
different ways, or in different places.

When lions and leopards live in the same area, for

example, lions take mostly larger animals as prey, and
leopards take smaller ones. Hawks and owls feed on
similar prey, but hawks hunt during the day and owls
hunt at night.

Figure 6-4 (p. 112) shows resource partitioning by

some insect-eating bird species. Figure 4-5 (p. 68)
shows how the evolution of specialized feeding niches
of bird species in a coastal wetland has reduced their
competition for resources.

6-3

How Do Species Interact?

C O N C E P T 6 - 3 A

Five basic species interactions—competition, predation, parasitism, mutu-

alism, and commensalism—affect the resource use and population sizes of the species in a
community.

C O N C E P T 6 - 3 B

Some species develop adaptations that allow them to reduce or avoid

competition for resources with other species.

83376_07_ch06_p105-122.ctp 8/10/07 12:18 PM Page 111

and have keen eyesight; still others, such as wolves and
African lions, capture their prey by hunting in packs.

Other predators use camouflage to hide in plain sight

and ambush their prey. For example, praying mantises
(Figure 3-1, left, p. 38) sit in flowers of a similar color
and ambush visiting insects. White ermines (a type of
weasel) and snowy owls hunt in snow-covered areas.
People camouflage themselves to hunt wild game and
use camouflaged traps to ambush wild game.

Some predators use chemical warfare to attack

their prey. For example, spiders and poisonous snakes
use venom to paralyze their prey and to deter their
predators.

Prey species have evolved many ways to avoid pred-

ators, including the abilities to run, swim, or fly fast,
and a highly developed sense of sight or smell that alerts
them to the presence of predators. Other avoidance
adaptations include protective shells (as on armadillos
and turtles), thick bark (giant sequoia), spines (porcu-
pines), and thorns (cacti and rosebushes). Many lizards
have brightly colored tails that break off when they are
attacked, often giving them enough time to escape.

Other prey species use the camouflage of certain

shapes or colors or the ability to change color
(chameleons and cuttlefish). Some insect species have
shapes that look like twigs (Figure 6-5a), bark, thorns,
or even bird droppings on leaves. A leaf insect can be
almost invisible against its background (Figure 6-5b),
as can an arctic hare in its white winter fur.

Chemical warfare is another common strategy. Some

prey species discourage predators with chemicals that
are poisonous (oleander plants), irritating (stinging net-

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CHAPTER 6

Community and Population Ecology

Blakburnian Warbler

Black-throated Green Warbler

Cape May Warbler

Bay-breasted Warbler

Yellow-rumped Warbler

Some Species Feed
on Other Species: Predation

In predation, a member of one species (the predator)
feeds directly on all or part of a living organism of
another species (the prey) as part of food webs (

Con-

cept 3-3

, p. 44). Together, the two kinds of

organisms, such as lions (the predator or
hunter) and zebras (the prey or hunted), form a
predator–prey relationship. Such relationships are
depicted in Figures 3-8 (p. 45) and 3-14 (p. 51).

At the individual level, members of the prey species

are clearly harmed. At the population level, predation
plays a role in evolution by natural selection. Predators,
for example, tend to kill the sick, weak, aged, and least
fit members of a population because they are the easi-
est to catch. This leaves behind individuals with better
defenses against predation. Such individuals tend to
survive longer and leave more offspring with adapta-
tions that help them avoid predation.

Some people tend to view predators with contempt.

When a hawk tries to capture and feed on a rabbit,
some root for the rabbit. Yet the hawk, like all preda-
tors, is merely trying to get enough food for itself and
its young. In doing so, it plays an important ecological
role in controlling rabbit populations.

Predators have a variety of methods that help them

capture prey. Herbivores can simply walk, swim, or fly
up to the plants they feed on. Carnivores feeding on
mobile prey have two main options: pursuit and am-
bush. Some, such as the cheetah, catch prey by running
fast; others, such as the American bald eagle, can fly

Figure 6-4 Sharing the wealth: resource partitioning of five species of insect-eating warblers in the spruce forests
of the U.S. state of Maine. Each species minimizes competition for food with the others by spending at least half its
feeding time in a distinct portion (shaded areas) of the spruce trees, and by consuming somewhat different insect
species. (After R. H. MacArthur, “Population Ecology of Some Warblers in Northeastern Coniferous Forests,”
Ecology 36 (1958): 533–536)

83376_07_ch06_p105-122.ctp 8/10/07 12:18 PM Page 112

(a) Span worm

(b) Wandering leaf insect

(c) Bombardier beetle

(d) Foul-tasting monarch butterfly

(f) Viceroy butterfly mimics
monarch butterfly

(e) Poison dart frog

(g) Hind wings of Io moth
resemble eyes of a much
larger animal.

(h) When touched,
snake caterpillar changes
shape to look like head of snake.

tles and bombardier beetles, Figure 6-5c), foul smelling
(skunks, skunk cabbages, and stinkbugs), or bad tasting
(buttercups and monarch butterflies, Figure 6-5d).
When attacked, some species of squid and octopus emit
clouds of black ink, allowing them to escape by confus-
ing their predators.

Many bad-tasting, bad-smelling, toxic, or stinging

prey species have evolved warning coloration, brightly
colored advertising that enables experienced predators
to recognize and avoid them. They flash a warning:
“Eating me is risky.” Examples are brilliantly colored
poisonous frogs (Figure 6-5e); and foul-tasting monarch
butterflies (Figure 6-5d).

Based on coloration, biologist Edward O.

Wilson gives us two rules for evaluating possible
danger from an unknown animal species we en-
counter in nature. First, if it is small and strik-
ingly beautiful, it is probably poisonous. Second,
if it is strikingly beautiful and easy to catch, it is
probably deadly.

Some butterfly species, such as the nonpoisonous

viceroy (Figure 6-5f), gain protection by looking and
acting like the monarch, a protective device known as
mimicry. Other prey species use behavioral strategies to
avoid predation. Some attempt to scare off predators by
puffing up (blowfish), spreading their wings (peacocks),
or mimicking a predator (Figure 6-5h). Some moths
have wings that look like the eyes of much larger ani-
mals (Figure 6-5g). Other prey species gain some pro-
tection by living in large groups such as schools of fish
and herds of antelope.

THINKING ABOUT

Predation and the American Alligator

What traits does the American alligator (

Core

Case Study

) have that helps it (a) catch prey and

(b) avoid being preyed upon?

Some Species Feed Off
Other Species by Living On or In
Them: Parasitism

Parasitism occurs when one species (the parasite)
feeds on the body of, or the energy used by, another
organism (the host), usually by living on or in the host.
In this relationship, the parasite benefits and the host is
harmed but not immediately killed.

Parasitism can be viewed as a special form of preda-

tion. But unlike the typical predator, a parasite usually
is much smaller than its host (prey) and rarely kills its
host. Also, most parasites remain closely associated
with their hosts, draw nourishment from them, and
may gradually weaken them over time.

Some parasites, such as tapeworms and some dis-

ease-causing microorganisms (pathogens), live inside
their hosts. Other parasites attach themselves to the
outsides of their hosts. Examples of the latter include

mosquitoes, mistletoe plants, and sea lampreys, which
use their sucker-like mouths to attach themselves to
fish and feed on their blood. Some parasites move from
one host to another, as fleas and ticks do; others, such
as tapeworms, spend their adult lives with a single host.

Some parasites have little contact with their host.

For example, North American cowbirds take over the
nests of other birds by laying their eggs in them and
then letting the host birds raise their young.

CONCEPTS 6-3A AND 6-3B

113

Figure 6-5 Some ways in which prey species avoid their predators:
(a, b) camouflage, (c, e) chemical warfare, (d, e) warning col-
oration, (f) mimicry, (g) deceptive looks, and (h) deceptive behavior.

83376_07_ch06_p105-122.ctp 8/10/07 12:18 PM Page 113

From the host’s point of view, parasites are harm-

ful. But at the population level, parasites can promote
biodiversity by increasing species richness and they
help to keep their hosts’ populations in check.

In Some Interactions, Both Species
Benefit: Mutualism

In mutualism, two species behave in a way that bene-
fits both by providing each with food, shelter, or some
other resources. For example, honeybees, caterpillars,
and other insects feed on a male flower’s nectar, picking
up pollen in the process, and then pollinate female
flowers when they feed on them. Coral reefs (p. 93) sur-
vive by a mutualistic relationship between reef-building
coral animals and bacteria that live in their tissues.

Figure 6-6 shows three examples of mutualistic re-

lationships that combine nutrition and protection. One
involves birds that ride on the backs of large animals
like African buffalo, elephants, and rhinoceroses
(Figure 6-6a). The birds remove and eat parasites and
pests (such as ticks and flies) from the animal’s body
and often make noises warning the larger animals
when predators approach.

A second example

involves clownfish spe-
cies, which live within
sea anemones, whose
tentacles sting and par-

alyze most fish that touch them and thus protect the
clownfish from some of its predators (Figure 6-6b). The
sea anemones benefit because the clownfish protect
them from some of their predators.

A third example is the highly specialized fungi that

combine with plant roots to form mycorrhizae (from
the Greek words for fungus and roots). The fungi get nu-
trition from the plant’s roots. In turn, the fungi benefit
the plant by using their myriad networks of hair-like
extensions to improve the plant’s ability to extract nu-
trients and water from the soil (Figure 6-6c and d).

In gut inhabitant mutualism, vast armies of bacteria in

the digestive systems of animals help break down (di-
gest) their host’s food. The bacteria receive a sheltered
habitat and food from their host. Hundreds of millions of
bacteria in your gut help you to digest the food you eat.

It is tempting to think of mutualism as an example

of cooperation between species. In reality, there is no
agreement between them to help one another. Instead,
each species benefits by unintentionally exploiting the
other as a result of traits they obtained through natural
selection.

In Some Interactions, One Species
Benefits and the Other Is Not Harmed

Commensalism is an interaction that benefits one
species but has little, if any, effect on the other. For ex-
ample, in tropical forests certain kinds of silverfish in-
sects move along with columns of army ants to share the
food obtained by the ants in their raids. The army ants
receive no apparent harm or benefit from the silverfish.

Another example involves plants called epiphytes

(such as certain types of orchids and bromeliads), which
attach themselves to the trunks or branches of large
trees in tropical and subtropical forests (Figure 6-7).
These air plants benefit by having a solid base on which
to grow. They also live in an elevated spot that gives
them better access to sunlight, water from the humid

114

CHAPTER 6

Community and Population Ecology

(a) Oxpeckers and black rhinoceros

(b) Clownfish and sea anemone

(c) Mycorrhizal fungi on juniper

seedlings in normal soil

(d) Lack of mycorrhizal fungi on

juniper seedlings in sterilized soil

Figure 6-6 Examples of mutualism. (a) Oxpeckers (or tickbirds) feed on parasitic ticks that infest large, thick-
skinned animals such as the endangered black rhinoceros. (b) A clownfish gains protection and food by living
among deadly stinging sea anemones and helps protect the anemones from some of their predators. (c) Beneficial
effects of mycorrhizal fungi attached to roots of juniper seedlings on plant growth compared to (d) growth of
such seedlings in sterilized soil without mycorrhizal fungi. (Oxpeckers and black rhinoceros: Joe McDonald/Tom
Stack & Associates; clownfish and sea anemone: Fred Beavendam/Peter Arnold, Inc.)

Luiz C. Marigo/Peter Arnold, Inc.

Figure 6-7 In an example of commensalism,
this bromeliad—an epiphyte or air plant in
Brazil’s Atlantic tropical rain forest—roots on
the trunk of a tree, rather than in the soil,
without penetrating or harming the tree. In
this interaction, the epiphyte gains access to
water, other nutrient debris, and sunlight; the
tree apparently remains unharmed.

83376_07_ch06_p105-122.ctp 8/10/07 12:18 PM Page 114

air and rain, and nutrients falling from the tree’s upper
leaves and limbs. Their presence apparently does not
harm the tree. Similarly, birds benefit by nesting in
trees, generally without affecting the trees in any way.

CONCEPTS 6-4A AND 6-4B

115

6-4

How Do Communities Respond to Changing
Environmental Conditions?

C O N C E P T 6 - 4 A

The structure and species composition of communities change in re-

sponse to changing environmental conditions through a process called ecological succession.

C O N C E P T 6 - 4 B

According to the precautionary principle, we should take measures to

prevent or reduce harm to human health and natural systems even if some possible cause-
and-effect relationships have not been fully established scientifically.

Review the way species can interact and see the

results of an experiment on species interaction at ThomsonNOW.

Time

Small herbs
and shrubs

Lichens and
mosses

Exposed
rocks

Heath mat

Jack pine,
black spruce,
and aspen

Balsam fir,
paper birch, and
white spruce
forest community

Communities and Ecosystems
Change over Time:
Ecological Succession

All communities change their structure and composi-
tion in response to changing environmental conditions
such as fires, climate change, or the clearing of forests
to plant crops. The gradual change in species composi-
tion of a given area is called ecological succession
(

Concept 6-4A

).

Ecologists recognize two types of ecological succes-

sion, depending on the conditions present at the begin-
ning of the process. Primary succession involves the
gradual establishment of communities in lifeless areas
where there is no soil in a terrestrial community
(Figure 6-8) or no bottom sediment in an aquatic com-
munity. Examples include bare rock exposed by a re-
treating glacier or severe soil erosion, newly cooled
lava, an abandoned highway or parking lot, and a
newly created shallow pond or reservoir.

Figure 6-8 Primary ecological succession: over almost a thousand years, plant communities developed starting on
bare rock exposed by a retreating glacier on Isle Royale, Michigan (USA), in northern Lake Superior. The details of
this process vary from one site to another.

Primary succession usually takes a long time—typi-

cally thousands or even tens of thousands of years.
Before a community can become established on land,
there must be soil. Depending mostly on the climate, it
takes natural processes several hundred to several thou-
sand years to produce fertile soil.

With the other, more common

type of ecological succes-
sion, called secondary
succession, a series

83376_07_ch06_p105-122.ctp 8/10/07 12:18 PM Page 115

116

CHAPTER 6

Community and Population Ecology

Time

Shrubs and
small pine
seedlings

Annual
weeds

Perennial
weeds and
grasses

Young pine forest
with developing
understory of oak
and hickory trees

Mature oak and hickory forest

Active Figure 6-9

Natural ecological restoration of disturbed land: secondary ecological succession

of plant communities on an abandoned farm field in the U.S. state of North Carolina. It took 150–200 years after the
farmland was abandoned for the area to become covered with a mature oak and hickory forest. A new disturbance,
such as deforestation or fire, would create conditions favoring pioneer species such as annual weeds. In the absence
of new disturbances, secondary succession would recur over time, but not necessarily in the same sequence shown
here. See an animation based on this figure at ThomsonNOW. Questions: Do you think the annual weeds (left)
would continue to thrive in the mature forest (right)? Why or why not?

of communities with different species can develop in
places containing soil or bottom sediment. This devel-
opment begins in an area where the natural community
of organisms has been disturbed, removed, or de-
stroyed, but some soil or bottom sediment remains.
Candidates for secondary succession include abandoned
farmlands (Figure 6-9), burned or cut forests, heavily
polluted streams, and land that has been flooded.
Because some soil or sediment is present, new vegeta-
tion can begin to germinate, usually within a few
weeks, from seeds already in the soil and from those
imported by wind or by birds and other animals.

During primary or secondary succession, distur-

bances such as fires, clear-cutting forests, plowing of
grasslands, or invasions by nonnative species can inter-
rupt a particular stage of succession, setting it back to
an earlier stage. Such disturbances create new condi-
tions that encourage some species and discourage or
eliminate others.

We tend to think of environmental disturbances as

harmful. But many ecologists contend that in the long
run, disturbances such as fires and hurricanes can be
beneficial for the species richness of certain communi-
ties and ecosystems. Such disturbances create new con-
ditions that can harm or eliminate some species, while
releasing nutrients and creating unfilled niches for
others. According to the intermediate disturbance
hypothesis, fairly frequent but moderate distur-
bances lead to the greatest species richness.

Primary and secondary ecological suc-

cession are important natural services
that tend to increase biodiversity and
thus the sustainability of communities
and ecosystems by increasing species
richness and interactions among
species. Such interactions in turn
enhance sustainability by pro-
moting population control and

by increasing the complexity of food webs for the en-
ergy flow and nutrient cycling that make up the func-
tional component of biodiversity (Figure 3-12, p. 48).

Explore the difference between primary and

secondary succession at ThomsonNOW.

Succession Doesn’t Follow
a Predictable Path

According to traditional view, succession proceeds in
an orderly sequence along an expected path until a
certain stable type of climax community occupies an
area. Such a community is dominated by a few long-
lived plant species and is in balance with its environ-
ment. This equilibrium model of succession is what
ecologists once meant when they talked about the bal-
ance of nature.

Over the last several decades, many ecologists have

changed their views about balance and equilibrium in
nature. Under the balance-of-nature hypothesis, a large

83376_07_ch06_p105-122.ctp 8/10/07 12:18 PM Page 116

terrestrial community undergoing succession eventu-
ally became covered with an expected type of climax
vegetation such as a mature forest (Figure 6-9). But a
close look at almost any community reveals that it con-
sists of an ever-changing mosaic of patches of vegeta-
tion at different stages of succession.

The current view is that we cannot predict the

course of a given succession or view it as preordained
progress toward an ideally adapted climax commu-
nity. Rather, succession reflects the ongoing struggle
by different species for enough light, nutrients, food,
and space. Most ecologists now recognize that mature
late-successional communities are not in a state of
permanent equilibrium, but rather a state of continual
disturbance and change.

Should We Protect Natural Systems
from Harmful Human Activities? The
Precautionary Principle

Some land developers argue that if we cannot predict
the course of succession and if nature is not in balance,
there is no point in trying to preserve and manage old-
growth forests and other ecosystems. They conclude
that we should cut down diverse old-growth forests, use
the timber resources, and replace the forests with tree
plantations of single-tree species (see photo 1, p. vi), a
food crop, or homes and other buildings.

Furthermore, they say, we should convert most of

the world’s grasslands to cropfields, drain and develop
inland wetlands, dump our toxic and radioactive wastes
into the deep ocean, and not worry about the premature
extinction of species. You can imagine that these ideas
make ecologists and conservation biologists go ballistic.

Ecologists point out that just because a system is not

in equilibrium or balance does not mean that it cannot
suffer from environmental degradation. They point to
overwhelming evidence that human disturbances
(Figure 1-6, p. 12, Figure 1-8, p. 13, and Supplement 4,
pp. S12–S22) are disrupting vital natural services that
support and sustain all life and all economies. They
contend that our uncertainty and unpredictability
about the effects of our actions means we need to use
great caution in making potentially harmful changes to
communities and ecosystems. They urge taking precau-
tionary action to help prevent potentially serious losses
of biodiversity.

This approach is based on the precautionary

principle: When substantial preliminary evidence
indicates that an activity can harm human health or
the environment, we should take precautionary meas-
ures to prevent or reduce such harm even if some pos-
sible cause-and-effect relationships have not been fully
established scientifically (

Concept 6-4B

). It is based on

the commonsense idea behind many adages such as
“Better safe than sorry,” “Look before you leap,” “First,
do no harm,” and “Slow down for speed bumps.”

The precautionary principle is a useful idea. But it

can be taken too far. If we don’t take some risks, we
will never learn what works and what doesn’t.

The message is that we should take some risks, possi-

bly disturbing some ecosystems, but always think care-
fully about the possible short- and long-term expected
and unintended effects. Using the precautionary princi-
ple comes in when the potential risks seem too great, or
when we don’t have much information about the possi-
ble risks. Then it is time to step back, think about what
we are doing, and do more research. Doing something
just because it can be done is not always a wise choice.

CONCEPT 6-5

117

Most Populations Live
in Clumps or Patches

Populations differ in factors such as distribution, num-
bers, and age structure (proportions of individuals in dif-
ferent age groups). Three general patterns of population
distribution or dispersion in a habitat are clumping, uniform
dispersion, and random dispersion (Figure 6-10, p. 118).

Individuals in the populations of most species live in

clumps or patches (Figure 6-10a). Examples are patches
of desert vegetation around springs, cottonwood trees
clustered along streams, wolf packs, and schools of fish.
The locations and sizes of these clumps vary with the
availability of resources.

Why clumping? There are four reasons: First, the re-

sources a species needs vary greatly in availability from
place to place. Second, living in groups protects some
animals from predators and therefore from population
declines. Third, living in packs gives some predator
species a better chance of getting a meal. Fourth, some
species form temporary groups for mating and caring
for young.

Some species maintain a fairly constant distance be-

tween individuals. Such a pattern gives creosote bushes
in a desert (Figure 6-10b) better access to scarce water
resources. Organisms with a random distribution (Fig-
ure 6-10c) are fairly rare. The living world is mostly
clumpy and patchy.

6-5

What Limits the Growth of Populations?

C O N C E P T 6 - 5

No population can continue to grow indefinitely because of limitations on

resources and because of competition among species for those resources.

83376_07_ch06_p105-122.ctp 8/10/07 12:18 PM Page 117

Populations Can Grow, Shrink,
or Remain Stable

Four variables—births, deaths, immigration, and emigra-
tion—govern changes in population size. A population
increases by birth and immigration (arrival of individu-
als from outside the population) and decreases by
death and emigration (departure of individuals from
the population):

Population

change

⫽ (Births ⫹ Immigration) ⫺ (Deaths ⫹ Emigration)

A population’s age structure—the proportion of

individuals at various ages—can have a strong effect on
how rapidly it increases or decreases in size. Age struc-
tures are usually described in terms of organisms not
mature enough to reproduce (the pre-reproductive stage),
those capable of reproduction (the reproductive stage),
and those too old to reproduce (the post-reproductive
stage).

The size of a population will likely increase if it is

made up mostly of individuals in their reproductive
stage, or soon to enter this stage. In contrast, a popula-
tion dominated by individuals past their reproductive
stage will tend to decrease over time. The size of a pop-
ulation with a fairly even distribution among these
three age groups tends to remain stable because repro-
duction by younger individuals will be roughly bal-
anced by the deaths of older individuals.

No Population Can Grow Indefinitely:
J-Curves and S-Curves

Species vary in their biotic potential or capacity for
growth. The intrinsic rate of increase (r) is the rate
at which a population would grow if it had unlimited
resources.

Some species have an astounding biotic potential.

For example, with no controls on their population
growth, bacteria that can reproduce every 20 minutes
would form a layer 0.3 meter (1 foot) deep over the
entire earth’s surface in only 36 hours.

Fortunately, this is not a realistic scenario. Research

reveals that no population can grow indefinitely because
of limitations on resources and competition between

118

CHAPTER 6

Community and Population Ecology

Clumped (elephants)

(a)

(b)

(c)

Uniform (creosote bush)

Random (dandelions)

Figure 6-10 Generalized dispersion patterns
for individuals in a population throughout
their habitat. The most common pattern is
clumps of members of a population through-
out their habitat, mostly because resources
are usually found in patches. Question: Why
do you think the creosote bushes are uni-
formly spaced while the dandelions are not?

species for those resources (

Concept 6-5

). In the real

world, a rapidly growing population reaches some size
limit imposed by one or more limiting factors, such as
light, water, space, or nutrients, or by exposure to too
many competitors, predators, or infectious diseases.
There are always limits to population growth
in nature. This is one of nature’s four

scientific

principles of sustainability

(see back cover and

Concept 1-6

, p. 19).

Environmental resistance is the combi-

nation of all factors that act to limit the growth of a pop-
ulation. Together, biotic potential and environmental
resistance determine the carrying capacity (K): the
maximum population of a given species that a particu-
lar habitat can sustain indefinitely without being de-
graded. The growth rate of a population decreases as its
size nears the carrying capacity of its environment be-
cause resources such as food, water, and space begin to
dwindle.

A population with few, if any, limitations on its re-

source supplies grows exponentially at a fixed rate such
as 1% or 2% per year. Exponential or geometric growth
(Figure 1-1, p. 5) starts slowly but then accelerates as
the population increases, because the base size of the
population is increasing. Plotting the number of indi-
viduals against time yields a J-shaped growth curve
(Figure 6-11, bottom half of curve).

Logistic growth involves rapid exponential popu-

lation growth followed by a steady decrease in popula-
tion growth until the population size levels off (Fig-
ure 6-11, top half of curve). This slowdown occurs as
the population encounters environmental resistance
and approaches the carrying capacity of its environ-
ment. After leveling off, a population with this type of
growth typically fluctuates slightly above and below the
carrying capacity.

A plot of the number of individuals against time

yields a sigmoid, or S-shaped, logistic growth curve (the
whole curve in Figure 6-11). Figure 6-12 depicts such a
curve for sheep on the island of Tasmania, south of
Australia, in the early 19th century.

Learn how to estimate a population of butter-

flies and see a mouse population growing exponentially at
ThomsonNOW.

83376_07_ch06_p105-122.ctp 8/10/07 12:18 PM Page 118

Some species do not make a smooth transition from

exponential growth to logistic growth. Such popula-
tions use up their resource supplies and temporarily
overshoot, or exceed, the carrying capacity of their envi-
ronment. This occurs because of a reproductive time lag:
the period needed for the birth rate to fall and the death
rate to rise in response to resource overconsumption.

In such cases, the population suffers a dieback, or

crash, unless the excess individuals can switch to new
resources or move to an area with more resources. Such
a crash occurred when reindeer were introduced onto a
small island in the Bering Sea (Figure 6-13, p. 120).

Species Have Different
Reproduction Patterns

Species use different reproductive patterns to help en-
sure their survival. Species with a capacity for a high
rate of population increase (r) are called r-selected
species (Figure 6-14, p. 120). These species have many,
usually small, offspring and give them little or no
parental care or protection. They overcome typically
massive losses of their offspring by producing so many
that a few will likely survive to reproduce many more
offspring to begin this reproductive pattern again.
Examples include algae, bacteria, rodents, annual plants
(such as dandelions), and most insects.

Such species tend to be opportunists. They reproduce

and disperse rapidly when conditions are favorable or
when a disturbance opens up a new habitat or niche for
invasion, as in the early stages of ecological succession.

Environmental changes caused by disturbances can

allow opportunist species to gain a foothold. However,
once established, their populations may crash because
of unfavorable changes in environmental conditions or
invasion by more competitive species. This helps explain
why most opportunist species go through irregular and
unstable boom-and-bust cycles in their population sizes.

At the other extreme are competitor or K-selected

species (Figure 6-14). They tend to reproduce later in
life and have a small number of offspring with fairly
long life spans. Typically the offspring of such species
develop inside their mothers (where they are safe), are
born fairly large, mature slowly, and are cared for and
protected by one or both parents, and in some cases by
living in herds or groups, until they reach reproductive
age. This reproductive pattern results in a few big and
strong individuals that can compete for resources and
reproduce a few young to begin the cycle again.

Such species are called K-selected species because

they tend to do well in competitive conditions when
their population size is near the carrying capacity (K) of
their environment. Their populations typically follow a
logistic growth curve (Figure 6-12).

Most large mammals (such as elephants, whales,

and humans), birds of prey, and large and long-lived
plants (such as the saguaro cactus, and most tropical
rain forest trees) are K-selected species. Ocean fish such

as orange roughy and swordfish, which are now being
depleted by overfishing, are also K-selected. Many of
these species—especially those with long times between
generations and low reproductive rates like elephants,
rhinoceroses, and sharks—are prone to extinction.

Most organisms have reproductive patterns be-

tween the extremes of r-selected and K-selected
species. In agriculture we raise both r-selected species
(crops) and K-selected species (livestock).

CONCEPT 6-5

119

Population size (

N

)

Time (t)

Environmental

resistance

Population stabilizes

Biotic
potential

Exponential
growth

Carrying capacity (K)

Year

1825

1800

1850

1875

1900

1925

Number of sheep (millions)

2.0

1.5

1.0

.5

Population
overshoots
carrying
capacity

Population
runs out of
resources
and crashes

Exponential
growth

Carrying capacity

Population recovers
and stabilizes

Figure 6-12 Logistic growth of a sheep population on the island of Tasmania between
1800 and 1925. After sheep were introduced in 1800, their population grew exponen-
tially thanks to an ample food supply. By 1855, they had overshot the land’s carrying
capacity. Their numbers then stabilized and fluctuated around a carrying capacity of
about 1.6 million sheep.

Active Figure 6-11

No population can continue to increase in size in-

definitely (

Concept 6-5

). Exponential growth (lower part of the curve) occurs when re-

sources are not limiting and a population can grow at its intrinsic rate of increase (r) or
biotic potential. Such exponential growth is converted to logistic growth, in which the
growth rate decreases as the population becomes larger and faces environmental resist-
ance. Over time, the population size stabilizes at or near the carrying capacity (K) of its
environment, which results in a sigmoid (S-shaped) population growth curve. Depend-
ing on resource availability, the size of a population often fluctuates around its carrying
capacity, although a population may temporarily exceed its carrying capacity and then
suffer a sharp decline or crash in its numbers. See an animation based on this figure at
ThomsonNOW. Question: What is an example of environmental resistance that hu-
mans have not been able to overcome?

83376_07_ch06_p105-122.ctp 8/10/07 12:18 PM Page 119

■

✓

THINKING ABOUT

r-Selected and K-selected Species

If the earth experiences significant warming during this cen-
tury as projected, is this likely to favor r-selected or K-selected
species? Explain.

RESEARCH FRONTIER

Calculating carrying capacity more exactly for various species
and ecosystems and for the earth

Humans Are Not Exempt
from Nature’s Population Controls

Humans are not exempt from population overshoot
and dieback. Ireland experienced a population crash af-
ter a fungus destroyed the potato crop in 1845. About

1 million people died, and 3 million people migrated to
other countries.

During the 14th century the bubonic plague spread

through densely populated European cities and killed
at least 25 million people. There is growing concern
that a global flu epidemic may kill hundreds of millions
of people.

Currently, the world is experiencing a global epi-

demic of eventually fatal AIDS, caused by infection
with the human immunodeficiency virus (HIV). Since
1980, AIDS has killed more than 25 million people and
claims another 3 million lives each year—an average of
6 deaths per minute. Between 2006 and 2050, the
World Health Organization estimates that AIDS will kill
at least 50 million more people, with the annual death
toll reaching 5 million per year.

So far, technological, social, and other cultural

changes have extended the earth’s carrying capacity for
the human species. We have increased food production
and used large amounts of energy and matter resources
to occupy normally uninhabitable areas. As humans
spread into other areas, they interact with and attempt
to control the populations of other species such as alli-
gators (

Core Case Study

) and white-tailed deer

in the United States (Case Study, below).

Some say we can keep expanding our ecological

footprint indefinitely mostly because of our techno-
logical ingenuity. Others say that sooner or later we
will reach the limits that nature always imposes on
populations.

HOW WOULD YOU VOTE?

Can we continue to expand the earth’s carrying capacity for
humans? Cast your vote online at www.thomsonedu
.com/biology/miller.

THINKING ABOUT

The Human Species

If the human species suffered a sharp population decline,
name three species that might move in to occupy part of our
ecological niche.

■

C A S E S T U D Y

Exploding White-Tailed Deer
Populations in the United States

By 1900, habitat destruction and uncontrolled hunting
had reduced the white-tailed deer population in the
United States to about 500,000 animals. In the 1920s
and 1930s, laws were passed to protect the remaining
deer. Hunting was restricted and predators such as
wolves and mountain lions that preyed on the deer
were nearly eliminated.

It worked, and to some suburbanites and farmers,

perhaps too well. Today there are 25–30 million white-
tailed deer in the United States. During the last
50 years, large numbers of Americans have moved
into the wooded habitat of deer and provided them

120

CHAPTER 6

Community and Population Ecology

Carrying
capacity

Year

1910

1920

1930

1940

1950

Number of reindeer

2,000

1,500

1,000

500

0

Population
crashes

Population
overshoots
carrying
capacity

Figure 6-13 Exponential growth, overshoot, and population crash of reindeer intro-
duced to the small Bering Sea island of St. Paul. When 26 reindeer (24 of them female)
were introduced in 1910, lichens, mosses, and other food sources were plentiful. By
1935, the herd size had soared to 2,000, overshooting the island’s carrying capacity.
This led to a population crash, when the herd size plummeted to only 8 reindeer by
1950. Question: Why do you think this population grew faster and crashed, unlike the
sheep in Figure 6-12?

r species;
experience
r selection

K species;
experience
K selection

K

Carrying capacity

Number of individuals

Time

Figure 6-14 Positions of r-selected and K-selected species on the sigmoid (S-shaped)
population growth curve.

83376_07_ch06_p105-122.ctp 8/10/07 12:18 PM Page 120

protect one area but cause the deer to seek food in
someone else’s yard or garden.

Deer can also be trapped and moved from one area

to another, but this is expensive and must be repeated
whenever deer move back into an area. Also, there are
questions concerning where to move the deer and how
to pay for such programs.

Should we put deer on birth control? Darts loaded

with a contraceptive could be shot into female deer to
hold down their birth rates. But this is expensive and
must be repeated every year. One possibility is an ex-
perimental single-shot contraceptive vaccine that
causes females to stop producing eggs for several years.
Another approach is to trap dominant males and use
chemical injections to sterilize them. Both of these ap-
proaches will require years of testing.

Meanwhile, people living in the suburbs can expect

deer to chow down on their shrubs, flowers, and gar-
dens unless they erect high deer-proof fences or use
other methods to repel them. Deer have to eat every
day just as we do. Suburban dwellers might consider
avoiding use of plants that deer like to eat.

THINKING ABOUT

White-Tailed Deer

Some blame the white-tailed deer for invading farms and sub-
urban yards and gardens to find food. Others say humans are
mostly to blame because they have invaded deer territory,
eliminated most of the predators that kept deer populations
down, and provided the deer with plenty to eat in their lawns
and gardens. Which view do you hold? Do you see a solution
to this problem? If so, what is it?

CONCEPT 6-5

121

R E V I S I T I N G

The American Alligator and Sustainability

The Case Study of the American alligator at the beginning of the
chapter illustrates the power humans have over the environment,
both to do harm and to make amends. As most American alliga-
tors were eliminated from their natural areas in the 1950s, scien-
tists began pointing out the ecological benefits these animals had
been providing to their habitats (such as building water holes,
nesting mounds, and feeding sites for other species). Scientific
understanding of these ecological connections led to protection
of this species and its recovery.

In this chapter, we have seen how interactions among organ-

isms in a community determine their abundances and distribution,
help limit population size, influence evolutionary change, and
help sustain biodiversity. We have also seen how communities re-
spond to changes in environmental conditions by undergoing
ecological succession. And we have explored how populations of

various species grow and shrink within their habitats’ carrying ca-
pacities based on natural limits to growth.

Biological communities are functioning examples of the four

scientific principles of sustainability

(see back cover) in action.

Populations of their species depend directly or indirectly on solar
energy and participate in the chemical cycling of nutrients. They
tend to develop and maintain a diversity of species to take advan-
tage of all available niches and to provide alternative paths for
energy flow and nutrient cycling. And a community’s populations
are controlled by interactions among its species, as well as by lim-
its imposed by its environment.

Chapter 7 applies the principles of population ecology, along

with the scientific principles of sustainability, to the human popu-
lation and its environmental impact.

with flowers, garden crops, and other plants they like
to eat.

Deer like to live in the woods for security and go to

nearby fields, orchards, lawns, and gardens for food.
Suburbanization has created an all-you-can-eat para-
dise for deer, and their populations in such areas have
soared. In some forests, they are consuming native
ground cover vegetation and allowing nonnative weed
species to take over. Deer also spread Lyme disease
(carried by deer ticks) to humans. In addition, each
year in the United States, 1.5 million deer–vehicle colli-
sions injure at least 14,000 people and kill at least 200
(up from 101 deaths in 1993).

There are no easy answers to the deer population

problem in the suburbs. Changing hunting regulations
to allow killing of more female deer cuts down the over-
all deer population. But these actions have little effect
on deer in suburban areas because it is too dangerous to
allow widespread hunting with guns in such populated
communities. Some areas have hired experienced and
licensed archers who use bows and arrows to help re-
duce deer numbers. To protect nearby residents, the
archers hunt from elevated tree stands and shoot their
arrows only downward. However, animal activists
strongly oppose killing deer on the ethical grounds that
hunting them is cruel and inhumane treatment.

Some communities spray the scent of deer preda-

tors or rotting deer meat in edge areas to scare off deer.
Others use electronic equipment that emits high-
frequency sounds, which humans cannot hear, for the
same purpose. Some homeowners surround their gar-
dens and yards with a high black plastic mesh fencing
that is invisible from a distance. Such deterrents may

We cannot command nature except by obeying her.

SIR FRANCIS BACON

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122

CHAPTER 6

Community and Population Ecology

R E V I E W Q U E S T I O N S

1. Explain why the American alligator is considered to be a

keystone species. Why do biologists consider its comeback
an important success story in wildlife conservation?

2. Define the term species diversity and distinguish between

species richness and species evenness. Explain how ecosystems
with high or low species diversity can both display stability.

3. Explain why the disappearance of an amphibian indicator

species, such as a frog, is a cause for ecological concern.

4. What are keystone species and how are they similar to, or

different from, a foundation species? Provide an argument
for the protection of sharks.

5. Provide two examples of the basic types of interactions

between species: interspecific competition, predation, par-
asitism, mutualism, and commensalism.

6. How does resource partitioning reduce competition for re-

sources among species?

7. Describe eight ways in which prey species can avoid their

predators.

8. Describe the ecological processes of primary and second-

ary succession.

9. What are the pros and cons of using the precautionary

principle to protect natural systems?

10. Explain the underlying ecological principles that support

the observation that no population can grow indefinitely.

C R I T I C A L T H I N K I N G

1. List three ways you could apply

Concept 6-4B

to make

your lifestyle and that of any children and grandchildren
you might have more environmentally sustainable.

2. Some homeowners in the U.S. state of Florida be-

lieve they should have the right to kill any alliga-
tor found on their property. Others argue against this no-
tion, saying alligators are a threatened species, and that
housing developments have invaded the habitats of alliga-
tors, not the other way around. Some would say the
American alligator has an inherent right to exist, regard-
less of how we feel about it. What is your opinion on this
issue? Explain. What would likely happen ecologically in
the areas where alligators live if they were all killed or re-
moved from those areas?

3. How would you experimentally determine whether

(a) an organism is a keystone species and (b) two bird
species feeding on the same plant are competing for the
same resources or are engaged in resource partitioning?

4. How would you respond to someone who claims it is not

important to protect areas of temperate and polar biomes
because most of the world’s biodiversity is found in the
tropics?

5. Use the second law of thermodynamics (

Con-

cept 2-4B

, p. 33) to help explain why predators

are generally less abundant than their prey.

6. How would you reply to someone who argues that (a) we

should not worry about our effects on natural systems be-
cause succession will heal the wounds of human activities
and restore the balance of nature, (b) efforts to preserve
natural systems are not worthwhile because nature is
largely unpredictable, and (c) because there is no balance
in nature, we should cut down diverse old-growth forests
and replace them with tree farms?

7. Explain why most species with a high capacity for popula-

tion growth (high biotic potential) tend to have a small
size (such as bacteria and flies) while those with a low ca-
pacity for population growth tend to be large (such as hu-
mans, elephants, and whales).

8. Why are pest species likely to be extreme r-selected

species? Why are many endangered species likely to be
extreme K-selected species?

9. In your own words, restate this chapter’s closing quota-

tion by Sir Francis Bacon. Do you agree with this notion?
Why or why not?

10. List two questions that you would like to have answered

as a result of reading this chapter.

L E A R N I N G O N L I N E

Log on to the Student Companion Site for this book at

www

.thomsonedu.com/biology/miller

and choose Chapter 6 for many

study aids and ideas for further reading and research. These in-
clude flash cards, practice quizzing, Web links, information on
Green Careers, and InfoTrac

®

College Edition articles.

For access to animations and additional quizzing, register and
log on to

at www.thomsonedu.com/thomsonnow

using the access code card in the front of your book. You can
also explore the

Active Graphing

exercises that your instructor

may assign.

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